1. Field of the Invention
The present invention relates to radionuclide-emitter, voltaic-junction batteries, and, more particularly, to compact electric batteries that are powered by the combination of a nuclear radiation emitting source and a responsive semiconductor voltaic-junction for service in many applications where chemical batteries are unsatisfactory or inferior.
2. The Prior Art
Compact long-life energy sources have wide applications in such fields as aerospace systems, cardiac pacemakers, computer memory maintenance, remote instrumentation, etc. Chemical batteries suffer generally from theoretical limits in the energy density that they can accommodate. Radionuclide-emmiter, voltaic-junction cells have much higher theoretical limits in energy density, in some cases more than a factor of 1,000 greater, but, in the past, have not achieved desirable high energy density and long life in practice. Major problems have been encountered in adapting such prior art cells for practical use at relatively low temperature.
Silicon p-n junction cells for directly converting radiation, either visible or ionizing, to electricity were developed in the early 1950's. Specific use of radio-isotopes to power silicon p-n cells, known as betavoltaic cells, were extensively studied in the 1970's for applications where low power but high energy density were important, for example, in cardiac pacemakers. A primary motivation for these studies was that the theoretical energy density is much higher in betavoltaic cells than in the best chemical batteries, 24.3 W-h/cm.sup.3 versus 0.55 W-h/cm.sup.3 for mercury-zinc batteries. Unfortunately, isotopes that could be employed with silicon had to be limited to low energy beta emitters because of radiation damage. For example, a typical threshold energy for electron damage is about 0.180 MeV assuming an atomic displacement damage threshold of 12.9 eV. Alpha particles were known to cause so much damage that they were not seriously considered at any energy. This constraint excluded the most potent nuclide sources, and thus restricted maximum power of such devices because of limits to the specific activity achievable at maximum concentration with reasonable half-lives.
In the aforementioned co-pending patent application of the inventors hereof, the invention is directed to a relatively powerful battery that operates at a temperature above the point at which damage is rectified by annealing in the voltaic-junction. In some applications, particularly some applications involving prosthetic inserts for the human body, batteries that operate at relatively low temperatures are required.